Optical differential phase-shift keying (DPSK) is a promising modulation format that offers high receiver sensitivity, high tolerance to major nonlinear effects in high-speed transmissions, and high tolerance to coherent crosstalk. In optical DPSK transmission, data information is conveyed by the optical phase difference between adjacent bits. Optical DPSK modulation includes differential binary phase shift keying (DBPSK), differential quadrature phase shift keying (DQPSK), and other related format variants.
To increase the capacity of optical transport networks, 40-Gb/s channels are being considered to replace the 10-Gb/s channels currently carried on these networks. Most of the core transport networks_are based on dense wavelength-division multiplexing (DWDM) with a channel spacing of 50 GHz, as specified by the International Telecommunication Union (ITU). This spacing is illustrated schematically in FIG. 1A, with Δfmin=50 GHz. Due to the much wider optical spectrum of a 40-Gb/s DBPSK signal, as compared to that of a typical 10-Gb/s signal, there is a severe penalty due to the tight optical filtering when transmitting the 40-Gb/s binary signals over the 50-GHz channel grid. This filtering penalty becomes even larger when multiple reconfigurable optical add/drop multiplexers (ROADMs) incorporating elements such as wavelength selective switches (WSS) are also inserted in the transmission system, as is common in most of today's transparent optical networks, such as that schematically depicted in FIG. 1B. For example, in the network 10 illustrated, a signal may be routed via multiple ROADM/WSS 11-14 between its origin 21 and its destination 22. This makes it difficult to transmit, with high spectral efficiency, such high-speed DPSK signals in a typical DWDM system.
An existing approach to achieve high spectral efficiency is to use bandwidth-efficient modulation formats such as DQPSK and duobinary. DQPSK, however, requires more complex and expensive transmitters and receivers, and duobinary has poorer receiver sensitivity than DBPSK and DQPSK.